1. Technical Field
The present invention relates to a method and an apparatus for measuring process errors, and more particularly to a method and an apparatus capable of measuring process errors including an overlay of photoresist patterns formed through a photolithography process.
2. Description of the Related Art
In order to improve the competitiveness in a semiconductor industry, unit processes ensuring the high productivity of semiconductor devices have been developed. In addition, methods and apparatuses for measuring process errors in each unit process are variously studied and developed. Particularly, process conditions are frequently varied when performing a photolithography process, which is one of main semiconductor manufacturing processes, so a process for controlling the variation of process conditions and an apparatus for performing the process are required.
One of problems in the photolithography process is a misalignment of a photoresist pattern formed through exposure and development processes. Recently, as semiconductor devices are highly integrated, an alignment margin is reduced and a diameter of a wafer is enlarged, so it is difficult to precisely align the photoresist pattern, so that the misalignment of the photoresist pattern becomes a serious problem. In order to prevent the misalignment of the photoresist pattern, optimizing an overlay measuring process is necessary, which is a process for checking the alignment of the photoresist pattern formed on the wafer.
According to a conventional overlay measuring process, after the formation of the photoresist pattern on the wafer W, an overlay between the photoresist pattern and a pattern layer formed therebelow is measured. At this time, pattern layers are complicatedly formed on each cell of the wafer, so it is difficult to precisely measure the overlay degree thereof. For this reason, alignment marks are added to scribe lines of the wafer W so as to measure the overlay degree. Generally, the overlay measuring process is carried out with respect to 30 to 40% of shots provided in the wafer. In addition, the alignment marks are positioned not at a local area of the wafer but over the whole area of the wafer.
Each overlay degree measured from alignment marks formed on the wafer is analyzed by using a regression analysis method so as to obtain overlay data. The overlay data represent a tilt degree or a rotation degree of the photoresist pattern in the X-axis or Y-axis direction with respect to the wafer and reticle.
After calculating an overlay correcting value by using the overlay data, the overlay correcting value is fed back to an exposure device, thereby preventing the misalignment failure of the wafer to be processed.
However, the conventional overlay measuring method has problems.
Firstly, it is difficult to prevent the misalignment failure in the whole area of the wafer though the overlay correcting value is fed back to the exposure device.
Specifically, even when the exposure process is carried out in the same condition with respect to the wafer, the wafer has an alignment region and a misalignment region thereon. It means that the overlay correcting value is precisely reflected in a predetermined region of the wafer so that misalignment failure is not present, but the overlay correcting value is not precisely reflected in the other region of the wafer so that misalignment failure occurs. The above phenomenon seriously increases as the diameter of the wafer is enlarged.
Secondly, since the overlay can be measured for one shot on each wafer, it is impossible to measure the overlay of abnormal shots (hereinafter, referred to as half-sized shots), which are positioned at an edge of the wafer and have half the size of a normal shot.
As the diameter of the wafer is enlarged, the number of half-sized shots formed at the edge of the wafer increases. However, the process is carried out without checking the overlay of the half-sized shots, so the failure rate of the wafer increases.